diff --git a/src/meshTools/sets/cellSources/surfaceToCell/surfaceToCell.C b/src/meshTools/sets/cellSources/surfaceToCell/surfaceToCell.C
index b9da7468d6..e476bc8348 100644
--- a/src/meshTools/sets/cellSources/surfaceToCell/surfaceToCell.C
+++ b/src/meshTools/sets/cellSources/surfaceToCell/surfaceToCell.C
@@ -142,7 +142,7 @@ bool Foam::surfaceToCell::differingPointNormals
                     pointToNearest
                 );
 
-            if (pointTriI != cellTriI)
+            if (pointTriI != -1 && pointTriI != cellTriI)
             {
                 scalar cosAngle = normals[pointTriI] & normals[cellTriI];